William M. Marshall and Matthew C. Deans

Glenn Research Center, Cleveland, Ohio

Recommended Figures of Merit for

Green Monopropellants

NASA/TM—2013-216560

August 2013

AIAA–2013–3722
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William M. Marshall and Matthew C. Deans

Glenn Research Center, Cleveland, Ohio

Recommended Figures of Merit for

Green Monopropellants

NASA/TM—2013-216560

August 2013

AIAA–2013–3722

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
Recommended Figures of Merit for Green Monopropellants

Hydrazine propellant has historically been used as a rocket thruster monopropellant 

since the mid-1960s. Mission managers are well aware of its characteristics and 

performance. However, it is a known toxic chemical and a wide effort is underway to reduce 

and/or eliminate its use worldwide. Several new propellant combinations have been 

developed in the last few years which tout or promise to provide same or better performance 

as hydrazine while being “non-toxic” or “green”. Yet, there is no consistent definition for 

what constitutes “non-toxic” or “green”, and thus no good figure of merit for which to 

compare. This paper seeks to review the three major categories of figures of merit, and 

discusses how they might be used to assess the viability of a propellant.

Nomenclature

APU = Auxiliary Power Unit

DACS = Divert and Attitude Control System

DOT = Department of Transportation

ECAPS = Ecological Advanced Propulsion Systems

EPA = Environmental Protection Agency 

E. U. = European Union

GHS = Globally Harmonized System

GSE = Ground Support Equipment

GRASP = Green Advanced Space Propulsion

HMIS = Hazardous Materials Identification System

KSC = Kennedy Space Center

MMH = Monomethyl Hydrazine

MSDS = Material Safety Data Sheet

NASA = National Aeronautics and Space Administration

NFPA = National Fire Protection Association

NTO = Nitrogen Tetroxide

OSHA = Occupational Safety and Health Administration

PPE = Personal Protective Equipment

PRISMA - Prototype Research Instruments and Space Mission technology Advancement

RCS - Reaction Control System

REACH - Registration, Evaluation, Authorization and Restriction of Chemical substances

William M. Marshall and Matthew C. Deans 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

NASA/TM—2013-216560 1
SCAPE - Self-Contained Atmospheric Protective Ensemble

TRL - Test Readiness Level

UDMH - Unsymmetrical Dimethyl Hydrazine

U. N. - United Nations

U. S. - United States

Isp - Specific impulse, s

ρ - Density, kg/m3 (lbm/ft3

) 

LD50 - Lethal Dose for 50 percent of population, mg/kg

Tc - Decomposition/chamber temperature, K (°R)

ΔV - Change in velocity, m/s (ft/s)

I. Introduction

In the last few years, interest has been gaining in less toxic replacements for hydrazine and its derivatives for inspace thruster systems.1-10 While bi-propellants offer some advantages in terms of handling and performance, the 

focus of this paper will be limited to monopropellant systems, because this is where much of the advancement in 

recent years has focused. In government and industry standards within the propulsion community there has not been 

a consistent definition of what constitutes “non-toxic” or “green”,

11 and this paper will seek to provide some key 

figures of merit to help in the selection of propellant formulations. The paper will look at the history of hydrazine, 

and what considerations must be taken when selecting a suitable replacement propellant with the aim of reduced 

toxicity, safer handling characteristics, and improved performance compared to current the hydrazine infrastructure.

A. Nomenclature

While the terms “non-toxic” or “green” propellants are common phrases that are used for the desired propellant 

alternatives to hydrazine, there is no real solid definition as to what these mean according to the various standards in 

existence. These standards will be discussed further, but the desire is to replace hydrazine with a propellant that has

significantly lower toxicity. Note that even the leading non-toxic propellant formulations being developed do not 

meet a wholly “non-toxic” definition by all the toxicity standards. Even commonplace chemicals like table salt 

(NaCl) have an inherent, albeit high threshold, level of toxicity.12 The goal is to replace hydrazine, which is seen not 

only as toxic but treated as likely carcinogenic in humans, with a propellant that will have a corresponding greater 

level of safety as well as a reduction in handling complexity and cost, while maintaining or improving performance. 

Claiming a “non-toxic propellant” or “green propellant” may be a misnomer because these terms imply benign or 

environmentally friendly characteristics. “Reduced toxicity propellants” or “lower toxicity propellants” are other 

common terms which are more accurate descriptors. However, since “green” is so commonly and widely used in 

the literature, this paper will continue to use that term as the descriptor for reduced toxicity propellants.

B. Historical Use of Hydrazine

Hydrazine (i.e. anhydrous hydrazine, N2H4) and its derivatives (monomethylhydrazine [MMH], unsymmetrical 

dimethylhydrazine [UDMH], etc.) have traditionally been used as rocket propellants since the early 1960’s. 

Hydrazine can either be paired with an oxidizer as a bi-propellant (as in the hydrazine – nitrogen tetroxide 

combination), or used alone with a catalyst as a monopropellant. Monopropellants are typically single component 

compounds which energetically decompose over a catalytic surface, as opposed to requiring some form of separate 

fuel/oxidizer combination and/or a spark ignition scheme. As early as 1973, the use of monopropellant hydrazine 

had become so common that thrusters using hydrazine as a propellant had been used almost exclusively compared to 

NASA/TM—2013-216560 2
any other propellant.13 Monopropellant hydrazine was first used on the Able-4 lunar spacecraft in 1959, and in flight 

in 1966 as part of the Titan I launch vehicle.14, 15

Hydrazine is an attractive propellant for several reasons. The development of the Shell 405 (later 

remanufactured by Aerojet as S-405) catalyst in the mid-late 1960’s provided rocket designers with a convenient 

way to decompose hydrazine16, 17 and has been recognized as being a significant contributor to the development of 

hydrazine monopropellant thrusters.13 Hydrazine as a monopropellant offers greater specific impulse than the coldgas thrusters developed prior to the mid-1960’s and offers a simpler system than bi-propellant thrusters since it only 

requires one set of tankage.

14, 15 Additionally, hydrazine also has a relatively low decomposition/chamber 

temperature (~800-900 K [1440-1620 °R]), meaning that low cost materials (such as stainless steels and aluminum 

alloys) may be employed for thruster chambers and components.

14, 18 Another advantage to hydrazine systems is a

historical familiarity. Prior to its mass development as a monopropellant, a single energetically decomposing liquid 

had not existed with the efficiency and capability that hydrazine possesses. Because of the development of the Shell 

405 catalyst/hydrazine thrusters, it rapidly became widely accepted through the rocket, missile, and energy 

generation communities. It has been a routinely selected propellant since the 1960’s. Due to the massive 

acceptance, a variety of hardware (thrusters, tanks, valves, etc.) have been qualified as propulsion system 

components.14, 15 This is particularly attractive to mission managers who must strive to buy down risk on spaceflight 

hardware so they have remained with the known propellant and heritage hardware. Incorporating a propulsion 

system and components which have not been extensively flight tested (Test Readiness Level (TRL) < 7-8) is seen as 

high risk for most missions. Though some of these green propellants have been developed for decades, a propulsion 

system using these would require flying not only a new propellant but a novel catalyst and other new components 

adapted to these propellants, most of which is assessed at TRL ~5 or below at present. Additionally, the existing 

knowledge base also applies in the handling procedures, Price14 notes that while hydrazine is not to be casually 

handled, that handling is “relatively straightforward”. There is already an extensive infrastructure in place for 

hydrazine fueling operations.

Since hydrazine has been in use for some time, its traits are well defined. Table 1 lists some of the properties of 

hydrazine.1, 14, 19 In order to gain the acceptance that hydrazine did, any alternative will have to meet or improve 

upon the properties and/or performance of hydrazine. While some of these parameters are pertinent to performance 

such as the density, specific impulse, and resulting density specific impulse, others are more directly related to the 

hazardous nature of hydrazine. For example, the vapor pressure of hydrazine is relatively low, meaning that it 

remains in liquid state in the event of a spill. This is attractive because it minimizes respiratory hazards in the event 

of spills. By contrast, the vapor pressure of ethanol, another common liquid propellant, is higher (0.82 psia) than for 

hydrazine (0.29 psia). Ensuring that material compatibilities, required thermal control, and vapor pressure of any 

hydrazine replacement are equal to or better than hydrazine could all help promote safety and make handling easier 

and more economical.

NASA/TM—2013-216560 3
C. Necessity for a Hydrazine Alternative

Despite the long history and knowledge based gathered with hydrazine, there has been a strong push to 

discontinue its use as a monopropellant. One of the chief disadvantages of hydrazine is toxicity. Hydrazine is 

classified as an E. U. toxic and environmental hazard (marked as “T” and “N" on placards, respectively), and in the 

U. S. is labeled by the National Fire Prevention Association (NFPA) Standard 704 as level 3 in health, fire and

reactivity.

20 The NFPA levels will be discussed in more detail in a following section, but health ratings above level 

3 mean that upon exposure it can cause both temporary and permanent debilitating effects with the potential of 

death. For hydrazine, this manifests in eye, throat, and nose irritation, blindness, headaches, nausea, pulmonary 

edema, seizures, and coma. It can severely damage the liver, kidneys, and central nervous system. It is corrosive and 

causes chemical burns with lasting severe dermatitis if exposed to the skin.21 Contrast this to ethanol, which is 

categorized by NFPA 704 as level 2 in health.22 Hydrazine is also recognized as a probable carcinogen that can 

cause cancer in humans upon exposure. One material safety data sheet (MSDS) for hydrazine notes that it may be 

fatal if absorbed through the skin,

20 while another notes that in the event of a spill, the area is to be evacuated 

immediately.

23 This second MSDS has a more severe NFPA 704 rating for hydrazine, labeling it as level 4 in 

health; however the level 3 ratings for health seem to be more consistent across the MSDS search. In either case, 

both MSDS contain strong language to avoid exposure. Europe is also strongly seeking to discontinue hydrazine 

usage through REACH and GRASP efforts.7

While hydrazine and other hypergolic propellants have many favorable storage and performance properties, 

recognized over 30 years of use, they are becoming much more problematic from a cost and handling perspective. 

As early as 1995, studies have been conducted to look at alternatives to these propellants.2

 Because of the now 

required self-contained hazardous material suits (e.g. SCAPE suits), extra procedural requirements, increased costs 

for hardware replacement due to corrosion, and other hazardous material handling equipment, ground costs for 

hydrazine operations are increasing.24 Compared to several other common propellants, hydrazine has the most 

stringent quantity distance requirements and accidental release measures.25 Additionally, the cost for bulk hydrazine 

has also increased significantly. Since 2002, costs for bulk hydrazine (anhydrous) have increased by 200 percent.

26 

Some propellants under current investigation as hydrazine replacements claim a greater than 2/3 cost savings in 

ground operation costs versus a similar sized hydrazine system.27, 28

II. Scope and Figures of Merit

There are numerous characteristics which may be viewed when comparing a propellant to the current state of 

the art with hydrazine. For this body of work, three major factors were considered: toxicity, handling and

transportation, and performance. Toxicity is a major concern, because it both increases the required personnel safety 

Table 1: Properties of hydrazine.

Parameter Value

Chemical Formula N2H4

Molecular Weight 32.05

Density @ 300 K, kg/m3 1004

Vapor Pressure, kPa (psia) @ 300 K 1.97 (0.29)

Specific Impulse, s 220 - 235

Density Specific Impulse, kg-s/m3 224 x 103

Compatible Materials 18-8 Stainless Steel, Most Aluminums, Most Titaniums, 

Teflon, Kel-F, Polyethylene, Butyl Rubber

Freezing Temperature, K (R) 275 (495)

Boiling Temperature, K (R) 387 (697)

NASA/TM—2013-216560 4
measures, but also concerning potential environmental impacts (hazards with spills or mishaps). Handling and 

transportation are concerns because the required costs and delays of ground operations can significantly contribute 

to monetary costs and schedule. Performance is a major concern of mission planners because it directly relates to the 

size and mass (and thus cost) of a given spacecraft. The higher performing a propellant, the smaller tanks

potentially required, and therefore the greater amount of mass savings for instrumentation. When speaking of 

figures of merit, these are the major areas on which to focus when comparing/selecting propellant options.

A. Toxicity

The term “non-toxic” has been used in touting the goals for replacing hydrazine yet this term is rarely defined in 

detail. Various definitions of toxicity all include references to substances of a chemical nature that can inflict 

debilitation, damage, or death upon entering an organism. More detailed definitions include a caveat along the lines 

of “in certain quantities or concentrations”. 

In quantifying a substance as toxic, the concentration issue becomes a key factor. There are a multitude of 

standards in existence generated by various agencies that address acute toxicity. These ratings may take into account 

considerations for their specific industries; for example the National Fire Protection Association (NFPA) fire 

diamond sets health hazard levels, which account for toxicity, based on the assumption that a chemical will be 

exposed to fire. Their standard is used to quickly recognize the level of threat present and determine the appropriate 

equipment and exposure duration that can be withstood. These levels largely serve as a representation of 

concentration limits that can be endured. To properly deal with these chemicals the respective material safety data 

sheets (MSDS) must be referenced to determine appropriate handling methods.29 A chemical may be categorized 

(as indicated on the MSDS) by a combination of toxicity standards, including various United States and European 

standards.

In terms of exposure duration, not only do these various standards have concentration thresholds (the amount of 

substance) but there is also a time-dependency (how long one can be exposed to the given concentration) associated 

with them. These standards have an assigned exposure period and rules for scaling concentrations proportionately 

with the exposure period. Table 2 shows a cross section of various standards for oral toxicity; the quantity that 

could be ingested orally before a fatal level is likely reached. The LD50 (Lethal Dose for 50 percent of the 

population) parameter that is given for these values is a commonly used parameter for all of these standards 

representative of the dose of the substance (typically in mg) that would be required per unit mass of a subject

(typically in kg) that would result in the death of 50 percent of the administered sample population. These 

populations are typically representative animal species such as mice, though testing has been administered to various 

other mammal and non-mammal species as well. There are other non-animal testing ways to parameterize the LD50

that have been developed in more recent years, though this has not been widely implemented. The acute oral toxicity 

values shown in Table 2 are adjusted for a four hour equivalent exposure.

The table also shows that not all of these standards have similar cutoffs or even a similar number of categories. 

The names of the categories also vary, with “toxic” labels being used across differing spans, and some systems using 

“categories” or “levels”. The highlighted category is the Globally Harmonized System (GHS) which is a project by 

the United Nations (U. N.) that seeks to create a uniform standard for all applications and countries to aid 

international commerce and ensure safety. This standard avoids labels in favor of a strictly categorized system, as 

seen in Table 3. This table shows not only the acute oral dosage limits but also the various other means of acute 

toxicity exposure; dermal, gases, vapors, and dusts & mists. All of the aforementioned various standards typically 

include breakdowns for each of these categories as well, further adding to the complexity of these systems.

NASA/TM—2013-216560 5
Generally a given category or level within a classification system will have a degree of effect associated with 

that level of exposure. For the highest toxicity levels, death is often a common effect. Some example statements 

from the Hazardous Materials Identification System (HMIS) and NFPA standards are stated in Table 4. To ground 

this discussion on toxicity, hydrazine falls in the 3rd level for this health and toxic aspect, according to the NFPA 

standards, and would thus be considered “highly toxic” by OSHA standards.20

Further complications in the adequate classification of new propellants are seen when the propellants are blends 

of multiple constituents. As the propellants are developed, a toxicity rating for the blend may not have been 

determined, but ratings for each constituent component may exist. However, the blended propellant may be less 

harmful overall than the individual components. For example, a propellant may be made less harmful due to 

dilution or the inability of a more harmful component to come out of solution. Initial safety measures may have to 

use the “worst case” ratings of the constituent components until the overall product is evaluated. A thorough 

Table 2: Comparison of acute toxicity standards31

NASA/TM—2013-216560 6
evaluation of these should be carried out with documented results. The benefit of reduced cost due to easier handling 

will only be realized if the propellant blends are rated safer.

Finally, in addition to acute toxicity, there are additional exposure hazard concerns. Among these are 

carcinogenicity, mutagenicity, reproductive toxicity, along with specific organ sensitivity and other dangers.30 

Hydrazine, for example, has been proven as a carcinogenic substance.20 In order to reduce risk, ensure safety, and 

enable potential cost savings, these hazards should be eliminated in future propellants.

B. Handling and Transportation

Due to the hazards of hydrazine, one of the most severe levels of personal protective equipment is required. 

The material safety data sheet (MSDS) for this propellant states that a full suit with face shield and respirator is 

required for the propellant and in the case of a large spill a full self-contained breathing apparatus is required.20 As 

common practice in the industry, Self-Contained Atmospheric Protective Ensemble (SCAPE) suits are worn to 

protect against the propellant and its toxic effects.24 Potential green propellants and their lower toxicity levels could 

have comparatively less requisite protective equipment.28 Some of the propellants under current evaluation require 

less stringent respiratory protection. Though they generally still require gloves and coveralls with goggles or face 

shields, they do not typically require a fully sealed suit.

Table 3: Acute toxicity categories for all means of exposure per U. N. Globally Harmonized System31

Table 4: Comparison of toxicity level thresholds

NASA/TM—2013-216560 7
Due to these less stringent handling requirements, spacecraft processing would not need to be suspended for 

fueling operations. Not only would there be a reduction in the monetary costs of operations, there would be a 

shortened schedule with each one of these tasks and there would be a corresponding lower time demand on 

personnel. With hydrazine, fueling operations require the evacuation of all personnel other than those directly 

involved in the fueling procedure.28 This means that time must be set aside specifically for these operations and all 

other work must cease. The ease of handling of green propellants would not only reduce delays caused by these 

operations but potentially allow other work to continue concurrently. A caveat to this is that the new propellants 

should be qualified to be to not introduce additional propellant loading or handling restrictions as compared to 

hydrazine. It should tolerate a similar or less restrictive environment with regards to static charge, mechanical shock 

and friction, ambient temperature regime, among other assembly, loading, and launch site concerns. 

One of the main advantages of replacing hydrazine, aside from the greater degree of safety to personnel, is in 

the possible reduced cost associated with ease of transport. Other studies have examined the resulting costs savings 

that can be incurred by the usage of reduced toxicity propellants as compared to their toxic predecessors.

4, 7

 These 

studies discuss how savings in time and direct costs can be achieved through the whole process of launching a 

spacecraft. With a less hazardous propellant, there is the potential for reducing costs by requiring a less complex 

propellant flow system, since a more toxic propellant requires more failure prevention devices and safety measures. 

Similarly, there would be a corresponding reduction in the required ground support equipment (GSE) and support 

personnel devoted to propellant handling operations. 

U. N. and Department of Transportation (DOT) packing and shipping policies are additional concerns that need 

to be minded. While some of the green propellants may fall in different classifications entirely from hydrazine (DOT 

hazard classification 8, corrosive, packing group I), acceptable shipping standards for these propellants must be 

achieved since this may still be considered a hazardous operation.

32 Acceptable shipping classifications are another 

potential point of cost savings that new propellants may achieve. There may not be an even basis to compare 

between shipping classifications, but preference should be given to propellants that can be transported by the most 

means and with less induced cost. Preference to less restrictive shipping reduces the need for special packaging and 

transportation modes, and thus can the propellant can be shipped more conventionally at lower costs. Each of the 

various hazard classes or divisions have three subset packing groups with ‘I’ being the most stringent and ‘III’ being 

least so, with the more stringent group requiring more specialized handling and greater associated costs. An 

improvement over this packing group is desired. Likewise, due to the shipping classifications that hydrazine falls 

under, it is forbidden to be transported by passenger rail or passenger air. In the case of cargo air, it is restricted to

quantities of 2.5 L. Given that spacecraft typically use several liters of propellant, this restriction severely limits 

transportation modes for hydrazine. Achieving a different classification can potentially enable transport on 

passenger modes or enable the air transport of larger quantities. This would allow for greater flexibility and 

potentially lower costs in the ground phase of a mission.

Additionally, due to the less stringent handling and safety requirements with green propellants, potentially 

lower bulk costs of the propellants may be achieved, since some of the high costs in hydrazine are due to the 

increased safety measures and protocols associated with large quantities.

7

 

C. Performance and Material Compatibility 

While toxicity and handling are the primary focus of interest in a hydrazine replacement, the performance of a

propellant cannot be overlooked. There are a number of methods to determine performance for a monopropellant. 

Three parameters are usually recognized as critical and they are specific impulse (Isp), density specific impulse 

(ρIsp), and decomposition or chamber temperature (Tc). Specific impulse and density specific impulse are useful for 

determining the size and mass of the required tankage and available ΔV for a given mission. Decomposition 

temperature determines the materials required (either catalyst or chamber) for thruster survivability. Goals by 

various parties have been stated that any hydrazine replacement has, as a minimum, the same or better performance 

NASA/TM—2013-216560 8
characteristics as compared to hydrazine.

33 The closer a propellant can perform to current hydrazine propulsion 

systems; the more readily it can be implemented as a drop-in replacement, saving development costs. The noted 

typical performance parameters for hydrazine are provided in Table 1. Other considerations of performance include 

response time (how long it takes for the propellant to combust once injected into the chamber) and any system 

considerations such as pre-heat requirements.

While the green propellants should be at minimum equal to hydrazine in all aspects, many of the current 

reported propellants under development show significantly increased performance over hydrazine despite having 

different heater or pulse characteristics. Thus while not equal in all aspects, improvements in specific impulse 

performance makes them very attractive as a hydrazine substitute. For a given impulse requirement, improved 

density specific impulse would require less volume of propellant. Thus, a potentially smaller propulsion system may 

be needed as compared to hydrazine. For a specific mission, this may enable larger payloads, longer mission life, or 

improved capability for the same amount of mass as a traditional hydrazine system.

With increased performance, green monopropellants may exceed the traditionally achieved performance range 

of hydrazine based monopropellants and even approach the ranges of hypergolic bi-propellants or even traditionally 

ignited space storable bi-propellants as detailed in Figure 1. These green propellants can be seen as not only 

replacing and expanding the performance of traditional monopropellants but also closing a performance gap.

Eliminating the need for secondary storage and flow systems required of bipropellants could allow for payload mass 

and capability enhancements. 

Requirements for catalyst bed heating should also be considered. Some of the ionic liquid monopropellants 

require pre-heated catalyst beds for proper and efficient combustion. Pre-heat requirements determine the amount of 

power draw required on satellite systems, which are power limited. The lower the catalyst pre-heat temperature the 

more quickly a system can be used on short notice which is attractive for applications such as Divert and Attitude 

Control Systems (DACS) or Reaction Control Systems (RCS). The state of the art in hydrazine propulsion, the 

Shell/S-405 catalyst, does not require any pre-heat and is capable of many spontaneous starts, even at temperatures 

as low as 273 K (492 °R).14, 16 Though catalyst temperature is one consideration, this is a potential trade with 

temperature regulation of the propellant tanks and flow systems. These are factors important in system architecture, 

but each propellant will have its own beneficial and hindering attributes.

As the current state of the art and most major developments in monopropellants are intrinsically dependent on a 

catalyst surface to function, the catalyst itself must be considered. While catalysts by their nature are not consumed, 

the potentially harsh chemical and thermal environments that are characteristic of monopropellant decomposition 

and combustion can cause the destruction of the catalyst through a variety of mechanisms.34 Since these new 

propellant systems will need to meet or exceed the performance of comparable hydrazine propulsion systems, a 

similar operational lifespan must be met and a similar range of duty cycles to the current performance of hydrazine 

on S-405 must be achieved.28 Likewise, there cannot be a significant change in total system mass due to a heavier 

flow system and/or catalyst that is not compensated by the increased propellant performance. The desire is for 

comparable vehicle system performance, though these new propellant and catalyst combinations may result in 

increased capability as well. Along with lifespan, other combustion related concerns like roughness and ignition 

delay should meet or exceed the current performance of hydrazine thrusters. These parameters are all typically 

dependent to some degree on the thrust class of the thruster and should be equated accordingly.

NASA/TM—2013-216560 9
There have been various efforts into igniting monopropellants without a catalyst. Any alternate form of ignition 

must likewise meet or exceed the lifespan, response, and duty cycle capabilities of current hydrazine systems.

Since many of the chemicals used as traditional monopropellants have been used in hypergolic bipropellant 

systems as well, many of the hypergolic bipropellants are also very toxic. Research into green hypergolic propellants

should also be an important consideration as well to further ensure safety, lessen cost, and potentially expand 

capability. Cost savings could be further accrued should the same propellant used as a monopropellant be used as a 

component in bi-propellant systems, since fewer handling systems would be required and handling operations could 

potentially be combined. 

III. Discussion and Recommendations 

As can be seen from the extensive review above, comparing and unifying the various standards, codes and 

performance expectations is complex and difficult. The goal of this paper is not to dictate industry standards or 

policies. However, an extensive review of the literature provides some view of the overall desires and suggestions 

for improvement. Thus, this paper seeks to make recommendations that cover most concerns. 

From a toxicity standpoint, efforts are under way to establish an international, universal standard, but those 

efforts are not complete and current standards are not always in agreement. Compounding the classification of new 

propellants comes when the propellants are blends of multiple constituents. As the propellants are developed, a 

toxicity rating for the blend may not have been determined, but ratings for each constituent component may exist. 

However, the blended propellant may be less harmful overall than the individual components due, for example, to 

dilution effects or the inability of the component to come out of solution. Initial safety measures may have to use 

the “worst case” ratings of the constituent components until the overall product is evaluated. A thorough evaluation 

of these should be carried out with documented results as the benefit of reduced cost due to easier handling will only 

be realized if the propellant blends are rated safer. Achieving a NFPA and HMIS health category level lower than 

Figure 1: Vacuum specific impulse performance ranges by propellant type – based upon 300 psia chamber 

pressure and nozzle expansion ratio of 40:1

NASA/TM—2013-216560 10
that of hydrazine is the desired goal. Any secondary health concerns (carcinogenic, mutagenic, etc.) should also be 

eliminated. 

With respect to handling and transportation, recognizing that any propellant is likely to be categorized as a 

hazardous material, improvements to the available means of handling and transportation is desired. For instance, a 

propellant that can be shipped in meaningful quantities by commercial air would be desired over one which is 

restricted on commercial air, since this would provide managers the greatest flexibility in transporting propellant for 

schedule sensitive operations. Additionally, a reduction in required PPE (such as eliminating SCAPE suit 

requirements) is highly desirable.

As a minimum, any propellant should have the same overall performance (Isp and ρ Isp) of hydrazine. 

Additionally, increasing chamber/catalyst lifespan and associated combustion properties like minimizing 

combustion delay and roughness are important goals, since this will increase the responsiveness and longevity for 

thruster replacements. The more potential for a system to be a drop in replacement for hydrazine systems, the more 

likely it will be adopted for future missions.

Table 5 sums up some of the pertinent goals concerning hydrazine replacements. Achieving these goals will not 

only yield a safer propellant but also enable efficiencies in cost and time. Note that these are goals, and it is 

recognized that a propellant may not meet all goals in the table. However, preference should be given to the 

propellants which meet the greatest set of goals. As a general rule of thumb, any propellant which would be used to 

replace hydrazine should meet two major criterion: 1.) It must have a performance (specific impulse or density 

specific impulse) as high or higher than hydrazine, and 2.) The ground handling considerations must be less 

hazardous (from a toxicity/safety viewpoint) and require less ground support equipment (from a personnel 

safety/handling viewpoint) than hydrazine.

Table 5: Propellant Replacement Goals

Metric Goal

Acute Toxicity/Health • NFPA, HMIS < 3

• OSHA < ‘Highly Toxic’

• GHS > 2

Exposure Hazards • Non-carcinogenic, non-mutagenic

Personal Protective Equipment (PPE) • Eliminate need for SCAPE suits

Transportation • DOT packing group II or III 

• Bulk quantities (e. g. 55 gal. drums) shippable by cargo rail/air; preference to those 

shippable by commercial rail/air

Performance • Isp > 220 - 230 s

• Reduce need for exotic materials

• Combustion delay and roughness comparable to hydrazine monopropellant systems for 

given thrust classes

• Lifespan comparable to hydrazine/S-405 monopropellant systems.

NASA/TM—2013-216560 11
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